Trunked communication systems are known. In a typical trunked system, a central controller allocates a limited number of communication channels among a plurality of subscriber units. Generally, a subscriber may be a mobile unit, portable unit or a control station. As used herein, a mobile unit is a trunked subscriber unit designed to be installed in a vehicle, a portable unit is a trunked subscriber unit designed to be carried on or about the person, and a control station is a trunked subscriber unit design to be permanently or semi-permanently installed in a building or other fixed location.
Each subscriber has an identification (ID) code, which, together with other information, comprises a call request that is transmitted to the central controller thereby indicating that the subscriber desires access to one of the communication channels. Normally, the call request is encoded with error correcting or other codes to form inbound signalling words (ISWs) to facilitate communications. The subscriber ISWs are transmitted to the central controller over a selected one of the communication channels. This channel is commonly referred to as the control channel. Generally, the particular communication channel used as the control channel is periodically changed as a control measure.
When not participating in a call, each subscriber monitors the control channel. When the central receives the call request ISW from a requesting (i.e., the calling) subscriber, it returns a channel grant outbound signalling word (OSW) if a channel is available. Upon receipt of the channel grant OSW, both the calling subscriber and the called subscriber(s) leave the control channel and move to the assigned communication channel to participate in the call. If, however, all the channels are in use, the central controller returns a "Busy" OSW and the subscriber must wait until a channel becomes available.
In addition to subscriber-to-subscriber calls, it is known to interconnect the trunked communication system with the public telephone system. Generally, the central controller must be programmed to recognize that the subscriber's ID is authorized for such interconnect service. In this way, an authorized trunked subscriber can receive and initiate interconnect calls with "land line" (i.e., private homes or offices) parties.
The communication range of a trunked system, while dependent on transmitter power and other factors, is generally twenty (20) miles (i.e., a system diameter of forty (40) miles). Within range, it is manifest that the mobile and portable subscribers may roam at will. If a subscriber should roam outside the range the trunked system, the subscriber would "lose" (i.e., be unable to receive) the control channel. In such cases, communication is not possible.
Clearly, contemporary trunked communication systems presuppose that intra-system roaming completely serves the needs of the subscribers. Generally, this assumption may be valid for the subscriber that travels locally between home and office, or from one local customer to the next. However, preventing inter-system roaming inherently limits and dis-serves the needs of several other types of subscribers. Examples of such other subscribers include, but are not limited to, state-wide, multi-stage, or regional sales, service, or law enforcement organizations, and medium or long range trucking or carrier companies. In fact, as a general statement, any subscriber that has a more than an infrequent need to travel beyond the communication range of its home trunking system could benefit by inter-system roaming.